This invention relates to the synthesis of various abed substituted sexithiophenes which have significant solubility in common organic solvents suitable for solvent casting and their use as the semiconducting channel in organic thin-film field-effect transistors.
Thin film transistors, known as TFT""s are widely used as switching elements in electronics, most notably for large area applications such as active matrix liquid crystal displays, smart cards etc. The Thin Film Transistor (TFT) is an example of a field effect transistor (FET). The best known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today""s conventional switching element for high speed applications.
Presently, TFT""s in most devices are made using amorphous silicon as the semiconductor. Amorphous silicon provides a less expensive alternative to crystalline siliconxe2x80x94a necessary condition for reducing the cost of transistors in large area applications. Application of amorphous silicon is limited to low speed devices, since its mobility (0.1-0.5 cm2/V*sec) is 15-20 thousand times smaller than that of crystalline silicon. Even though amorphous silicon is cheaper to deposit than highly crystalline silicon, deposition of amorphous silicon requires relatively costly processes, such as plasma enhanced chemical vapor deposition, and high temperatures (xcx9c360xc2x0 C.) to achieve electrical characteristics sufficient for display applications.
In the past decade organic semiconductors have received much attention as potential semiconductor channels in TFTs, for example, U.S. Pat. No. 5,347,144 to Garnier et al., entitled xe2x80x9cThin-Layer Field-Effect Transistors with MIS Structure Whose Insulator and Semiconductors Are Made of Organic Materialsxe2x80x9d. Organic materials (small molecules, short-chain oligomers and polymers) may provide a less expensive alternative to inorganic materials (e.g., amorphous silicon) for TFT applications as they are simpler to process, especially those that are soluble in organic solvents and therefore can be applied to large areas by far less expensive processes, such as spin-coating, dip-coating and microcontact printing. Furthermore organic materials may be deposited at low temperatures opening up a wider range of substrate materials including plastics for flexible electronic devices.
Several short-chain and oligomeric organic materials have been synthesized (e.g., xcex1-sexithiophene) and have demonstrated mobilities close to amorphous silicon (0.1-0.6 cm2/V*sec); however, these relatively high mobilities have only been achieved by high-temperature vacuum deposition, since most of these compounds are not soluble in organic solvents. Some soluble long-chain organic compounds (e.g., polyalkylthiophenes) have been synthesized which have mobilities of 0.001-0.01 cm2/V*sec, but these materials usually have low on-off ratios, they must be applied under an atmosphere of inert gas and they must be extensively treated with base to reduce unintentional dopants introduced during polymerization to show semiconducting effects.
Accordingly, it is an object of this invention to synthesize soluble derivatives of the oligomer sexithiophene which are symmetrically substituted at the xcex1- and xcfx89- positions with various functional groups.
It is another object of this invention to use these derivatives of sexithiophene as low-cost, low-temperature alternatives to amorphous silicon as the semiconducting component in TFT devices.
A broad aspect of the present invention are soluble derivatives of sexithiophene in which terminal carbons are substituted with various polar groups such as phosphonic esters, phosphonic acids, phosphonates, carboxylic acids, carboxylates, mines, amides, carbamates, and alcohols, each separated from the terminal thiophene rings by one or more methylene groups, are synthesized. An TFT device in accordance with the second objective of this invention employs films of the above sexithiophene derivatives as the semiconducting component. These organic semiconductors are dissolved in common organic solvents and applied to the surface of a substrate using inexpensive, low-temperature solution-based processing such as spin-coating, dip-coating, drop-casting, or microcontact printing.